Acidic buffers

It is important to provide a stable pH in aquaria, and this can be achieved by pH buffers. pH buffers form equilibria that shift to oppose the pH change when a small amount of acid or base is added to the solution, maintaining a stable pH value.

Theory:

Calculation:

Acidic buffers are a mixture of a weak acid and a soluble salt of the acid.

Buffers follow the following equation

where:

  • Ka is the acid dissociation constant (the calculated strength of the acid).
  • [H+] is the concentration of dissociated protons.
  • [A-] is the concentration of the deprotonated acid species.
  • [HA] is the concentration of the protonated acid species.

To calculate how to produce an acidic buffer, the formula must be rearranged to find the relative quantities of protonated acid to deprotonated acid.

K/[H+] = [A-] / [HA]

Now the desired pH value and used species must be decided upon. I will aim for a pH of 6 and use the species CH3COOH, ethanoic acid.

K/[H+] = [CH3COO-] / [CH3COOH]

pH = -log[H+], therefore [H+] = 10^-6

The acidity (pKa) of ethanoic acid is 4.792. pK= -logKa, therefore K= 10^-4.792

If these values are substituted into the rearranged formula…

(1.61×10^-5)/(10^-6)= 161/10 = [CH3COO-] / [CH3COOH]

161 [CH3COO-] = 10 [CH3COOH]

Method:

To physically create the example buffer for each litre of aquarium, you will need:

  • Pure sodium hydroxide NaOH
  • Pure aqueous ethanoic acid CH3COOH
  • Pure water (reverse osmosised, distilled, clean rainwater)

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